How to Log HTTP Responses in Go with Middleware Chaining?

Logging Responses to Incoming HTTP Requests in http.HandleFunc

In response to the question posed in a previous thread, this article proposes logging HTTP responses from live servers without resorting to faking requests. The goal is to capture both headers and JSON payloads for thorough analysis.

Utilizing the concept of middleware chaining, we can create a custom HTTP handler that intercepts responses and logs relevant information. Here's how:

Middleware Chaining Approach

With middleware chaining, you define middleware functions that can execute before or after the main handler function. These middlewares can perform various tasks such as logging, validation, or security checks.

Implementing Functional Handler Combinators

Instead of magical negroni solutions, we can define our own functional handler combinators. Each combinator handles a specific task and can be combined to form a chain of execution. For instance, you could create a combinator for response logging:

<code class="go">func NewResponseLoggingHandler(next http.HandlerFunc) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        // Replace response writer with a recorder for subsequent handlers
        c := httptest.NewRecorder()
        next(c, r)

        // Copy response from recorder to actual response writer
        for k, v := range c.HeaderMap {
            w.Header()[k] = v
        }
        w.WriteHeader(c.Code)
        c.Body.WriteTo(w)
    }
}</code>

Managing Middleware Chains

To apply the logging middleware to all handlers in a specific category, we can create a default handler:

<code class="go">func NewDefaultHandler(next http.HandlerFunc) http.HandlerFunc {
    return NewResponseLoggingHandler(NewOtherStuffHandler(next))
}</code>

This combinator ensures that all chains created via NewDefaultHandler will include response logging along with any other default functionality defined in NewOtherStuffHandler.

By leveraging these techniques, you can seamlessly log HTTP responses from your live web service, providing valuable insights for debugging and analysis.

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